An electrical oscillator is a circuit that produces an electrical signal at a frequency defined by a time constant specific to it. FIG. 1 illustrates a principle diagram for an electrical oscillator that includes an inductance L, a capacitor C and a holding amplifier A. The oscillator time constant is equal to √{square root over (LC)} and the oscillation frequency is equal to ½π√{square root over (LC)}.
The amplitude of voltage oscillations measured at the terminals of the LC circuit is determined by non-linearities of the holding amplifier. It is important to check the amplitude of the voltage oscillations to achieve correct interfacing between the oscillator and the oscillator load circuits that receive the oscillation voltage on their inputs.
If the oscillator is integrated on silicon, manufacturing parameter dispersions affect the value of the amplitude of oscillations. For example, this is the case for the resistivity of metals for which the dispersions (+/−10%) modify the quality coefficient of the inductances and the capacitors, or the resistivity of poly-crystalline silicon for which the dispersions (+/−20%) modify the oscillator biasing current.
One known way of reducing these dispersions is to make amplitude slaving circuits that apply a retroaction on the oscillator biasing current. For example, this type of slaving circuit is described in the article titled “A 2V 2.5 GHz-104 dB/Hz At 100 kHz Fully Integrated VCO Wideband Low Noise Automatic Amplitude Control” (Alfio Zanchi et al, IEEE JSSC, VOL 36, No. 4, pp. 611-619, April 2001), and in the article titled “A Low Noise Low Power VCO With Automatic Amplitude Control For Wireless Application” (M. A. Margarit et al., IEEE JSSC, VOL 34, No. 6, pp. 761-771, June 1999).
An example of an oscillator with its slaving circuit according to known art is shown in FIG. 2. The circuit in FIG. 2 comprises an oscillator 1, a rectification stage 2 and a differential amplifier 3. The oscillator 1 comprises two transistors Q1, Q2, two resistances R1, R2, two inductances L1, L2, three capacitors C1, C2 and C3, and a current generator G. Three biasing voltages VBB, VCC, VEE power the oscillator 1. The output voltage from the oscillator 1 is taken from the terminals of capacitor C1.
The rectification stage 2 receives the output voltage from the oscillator on its input and detects the peak level of the oscillation voltage, for example, by double alternation rectification. The output voltage from the rectification stage is applied to a first input to the differential amplifier 3. A reference voltage Vref is applied to the second input of the differential amplifier 3. The output signal from the differential amplifier 3 controls the amplitude of the biasing current that passes through the current generator G. Thus, the amplitude of the output voltage from the oscillator 1 is controlled by the level of the reference voltage Vref.
This type of circuit has the disadvantage that it copies all the noise of the reference voltage Vref for frequencies within the slaving passband, and that there is no noise control outside this passband. This copying affects the phase noise in addition to the amplitude noise. Ideally, amplitude regulation should only generate an amplitude modulation. However, the non-linear nature and parametric effects of the oscillator cause the amplitude noise to be converted to phase noise, thus deteriorating the spectral quality of the oscillation signal.